The present invention relates to a binary coding circuit for converting analog electrical signals outputted from a photoelectric conversion device into binary signals representative of a background region and a character region in which fluctuations in characteristic of photoelectric conversion elements forming the photoelectric conversion device and adverse effects due to inclination of the photoelectric conversion device are corrected.
In general, in an optical character reading device such as a symbol reading device, a sheet having characters or symbols, hereinafter referred to as "characters or the like" when applicable, is scanned with a photoelectric conversion device to provide analog electrical signals. The analog electrical signals are converted into binary signals representative of a background region and a chamber region by a binary coding circuit so that the characters or the like may be recognized using the binary signals. In this operation, the threshold value for the binary coding operation is fixed. Therefore, in order to correctly recognize characters or the like, it is essential that the levels of the analog electrical signals outputted by the photoelectric conversion device have predetermined values with respect to the background region and the character region. It is ideal that all the photoelectric conversion elements forming the photoelectric conversion device are equal in sensitivity characteristics. However, in practice, the various photoelectric conversion elements are not uniform in their sensitivity characteristics. Accordingly, for binary coding the analog electrical signals, the analog electrical signals representative of the background region may be detected as signals representative of the character region or vice versa. In addition, in cases where the refractive index of a part of the sheet is different from that of another part, the light source does not provide uniform illumination, or the sheet is tilted with respect to the light receiving surface of the photoelectric conversion device, it may be impossible to convert the output analog electrical signals of the photoelectric conversion device into binary signals which are correctly representative of the background region and the character region.
Accordingly, an object of the invention is to eliminate the above-described difficulties. More specifically, an object of the invention is to provide a binary coding circuit in which not only disadvantageous effects due to the fluctuations in characteristics of the photoelectric conversion elements but also effects due to the fluctuations in general sensitivity characteristics including the illumination distribution characteristic of the light source and the lens system are corrected for and adverse effects due to inclination of the light receiving surface of the photoelectric conversion device are also corrected for.
Additionally, in a conventional binary coding circuit, output analog electrical signals from the photoelectric conversion device are compared with a fixed reference voltage to obtain the corresponding binary signals. Since a fixed reference voltage is employed, when the levels of the analog electrical signals provided by the photoelectric conversion device are varied because of variations in intensity of the light source or due to a non-uniformity in the reflection factor of the sheet, the outputted binary signals are often erroneous. In other words, since the reference voltage cannot follow the variations in level of the analog electrical signals, a character region may be detected as the background region or vice versa.
In order to overcome the above-described drawback, a binary coding circuit has been proposed in which a device for detecting the maximum value and/or the minimum value of the analog electrical signals for single scanning line or one scanning column among the output analog electrical signals of the photoelectric conversion device is provided with which the analog electrical signals are converted into a binary signal with the maximum value and/or the minimum value thereof detected as a threshold value for the single line or column. That is, in the binary coding circuit, the threshold value is changed whenever one line or one column is scanned. Therefore, the provision of erroneous binary signals due to variations in intensity of the light source or a non-uniformity in reflection factor of the sheet having characters to be read can be prevented.
However, the conventional binary coding circuit is still disadvantageous in that, if the sheet is inclined with respect to the light receiving surface of the photoelectric conversion device in a scanning direction, erroneous binary signals may be outputted. For instance, in a hand-held scanning type optical character reading device, the light receiving surface of the photoelectric conversion device may unavoidably be inclined with respect to the sheet.
In view of the foregoing, a further object of this invention is to provide a binary coding circuit in which even when the light receiving surface of the photoelectric conversion device is inclined with respect to the sheet to be read in the direction of scanning, the analog electrical signals produced by the photoelectric conversion device are converted into binary signals correctly representative of the character region and the background region of the sheet.